Metastatic breast cancer is a chemotherapy-responsive disease, and significant palliation of cancer-related symptoms can be achieved with effective treatment. New treatments are needed because patients with metastatic breast cancer commonly out-live the effectiveness of currently available cytotoxic and hormonal treatments. Pemetrexed is a novel antimetabolite that inhibits three enzymes critical in purine and pyrimidine biosynthetic pathways: thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyltransferase. Several phase II studies of pemetrexed have showed objective response rates of more than 30% in minimally pretreated metastatic breast cancer patients and approximately 20% in more heavily pretreated patients. Pemetrexed is associated with limited toxicity when administered with folic acid and vitamin B(12) supplementation and is therefore a promising agent both for palliative treatment of metastatic disease and for incorporation into combination regimens for treating newly diagnosed metastatic and early-stage breast cancer